1. Field of the Invention
The present invention relates to a swash plate type compressor having double headed pistons, and in particular, it relates to an improved thrust bearing arrangement for supporting an axial load in a swash plate type compressor.
2. Description of the Related Art
Swash plate type compressors are often used in refrigerating systems in vehicles. The swash plate type compressor disclosed in Japanese Unexamined Patent Publication (Kokai) No. 64-63669, for example, is one in which a swash plate is fixed to a drive shaft which is rotatably supported in a cylinder block by front and rear radial bearings and front and rear thrust bearings. The cylinder block comprises axially arranged front and rear cylinder block halves, and front and rear housings are attached to the cylinder block. The front thrust bearing is arranged between the front inner wall portion of the cylinder block and the boss of the swash plate, and the rear thrust bearing is arranged between the boss of the swash plate and the rear inner wall portion of the cylinder block. The front and rear cylinder block halves and the front and rear housings are fastened together by fastening means such as fastening bolts, and the front and rear thrust bearings are simultaneously fastened between the boss and the wall portions by the fastening bolts.
The thrust bearing comprises an inner ring, an outer ring, rollers, and a retainer. A first annular supporting projection having a larger diameter is provided on each of the opposite surfaces of the boss of the swash plate so that the inner ring of the thrust bearing abuts against the first annular supporting projection. A second annular supporting projection having a smaller diameter is provided on each of the opposite surfaces of the wall portion of the cylinder block so that the outer ring of the thrust bearing abuts against the second annular supporting projection. Each of the thrust bearings is pinched between the first and second annular supporting projection and can be elastically deformed due to the difference of the diameters of the first and second annular supporting projections when the fastening bolts are tightened to assemble the compressor, whereby a variation in an axial fastening length or fastening force of the fastening bolts is absorbed by the elastic deformation of the thrust bearings.
However, in this thrust bearing arrangement, the inner and outer rings, the rollers, and the retainers are subjected to a relatively strong load, and therefore, a very high quality control is requested. Also, the life of the thrust bearings may be shortened and the reliability of the compressor may be affected.
The swash plate type compressor also includes an electromagnetic clutch by which the compressor is driven from an engine of a vehicle. When the electromagnetic clutch is turned on and the compressor is driven, the coolant gas is compressed and a compression reaction force acts on the swash plate. The compression reaction force is then transmitted to and supported by the thrust bearings. The elastically deformable arrangement of the thrust bearings due to the difference of the diameters of the first and second annular supporting projections, can be considered to be an equivalent of an arrangement in which spring elements are arranged on either side of the swash plate. The compression reaction force induces a moment on the swash plate, and the spring elements arranged on either side of the swash plate interfere with each other to cause the swash plate to oscillate unstably. The oscillation may cause noise, including audible high frequency components, to appear during high speed rotation.
Also, Japanese Unexamined Utility Model Publication (Kokai) No. 54-170410 discloses a swash plate type compressor, in which the boss of the swash plate and the inner wall portion of the cylinder block have facing flat supporting seats, and each thrust bearing is rigidly arranged between the facing flat supporting seats of the boss and the wall portion of the cylinder block. However, in this arrangement, a management of an axial fastening length, i.e., a fastening force, of the fastening bolts is very difficult. If a member fastened by the fastening bolts is made of aluminum alloy, it is impossible to absorb a variation in a fastening force derived from thermal expansion of aluminum alloy.